Hybrid Clean Approach for Post-Copper CMP Defect Reduction Wei-Tsu Tseng,* Vamsi Devarapalli, James Steffes, Adam Ticknor, Mahmoud Khojasteh, Praneetha Poloju, Colin Goyette, David Steber, Leo Tai, Steven Molis, Mary Zaitz, Elliott Rill, Surbhi Mittal, Michael Kennett, Laertis Economikos, George Ouimet, Christine Bunke, Connie Truong, Stephan Grunow, Michael Chudzik IBM Semiconductor Research & Development Center, Hopewell Junction, NY 12533, USA *Corresponding author:
[email protected]
small alumina (Fig. 2a) or silica (Fig. 2b) residual abrasive particles in CMP slurries tend to cluster together due to their high surface charge to volume ratio. As a consequence, such residual abrasives become more difficult to clean up than larger PR or FM.
Abstract— A “hybrid” post-Cu CMP cleaning process that combines acidic and basic cleans in sequence is developed and implemented. The new process demonstrates the advantages of both acidic and basic cleans and achieves a more than 60% reduction in CMP defects, such as polish residues, foreign materials, slurry abrasives, scratches, and hollow metal, relative to an all-basic brush clean process. It also eliminates the circular ring defects that occur intermittently during roller brush clean. TXRF scans confirm the reduction of AlOx defects when using the hybrid clean process. XPS spectra show similar Cu surface oxidation states between the basic and hybrid clean processes. Both short and open yields can be improved by using the new clean process. The underlying mechanism of the huge defect reduction benefits is discussed.
Fig. 1: Circular ring defects resulting from the scrubbing during roller brush cleaning.
Keywords- defect reduction; Cu CMP; post clean.
I. INTRODUCTION For semiconductor manufacturing with Cu interconnects, the defects associated with Cu chemical-mechanical planarization (CMP) process are quite often the main yield detractor.1 Since CMP is the final and enabling process before one level of Cu interconnects is fully defined, not only can it generate defects during the process per se (e.g., scratches and polish residues), but it can also reveal or decorate the defects generated from prior process steps, such as post-RIE cleaning, liner deposition, and Cu plating. Therefore, not only must the post Cu CMP cleaning process remove the defects created during CMP, it also needs to be compatible with prior processes to prevent exacerbating the defects incoming to CMP.
Fig. 2: (a) Residual alumina abrasive (left); and (b) cluster of silica abrasives (right).
Historically, acidic chemicals were commonly adopted to remove oxide or particulate defects. Nowadays, however, most of the post-Cu CMP clean chemicals for advanced technology nodes operate in the neutral to high pH regimes in order to passivate the Cu surface more effectively and prevent corrosion-related defects. Inhibitors, surfactants, or chelating agents are often added and a repulsive zeta-potential is built in to enhance the cleaning efficiency of PR/FM type of defects. In this contribution, we investigate the advantages of both acidic and basic clean chemicals and develop a hybrid clean process3 by implementing the two different clean chemistries together to meet the Cu CMP cleaning challenges.
Polish residues (PR), polish scratches (PS), and foreign materials (FM) are the most common Cu CMP defects. In addition, corrosion-related defects such as hollow metal (HM) and Cu dendrites (DE) can be observed post Cu CMP.2 With the shrinking design rule, new types of CMP defects emerge in advanced technology nodes such as 32nm and beyond. Among these, circular ring defects (sometimes referred to as brush scrubbing scratches3) are unique in that they are defects generated during the brush cleaning step with distinct concentric circle signatures that follow the path of particles in motion on roller brushes, as shown in Fig. 1. In addition, the
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II. EXPERIMENTAL All the CMP work in this study is conducted on 300mm CMP polishers, each equipped with a standard megasonic tank
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and roller brush stations. Wafers with Cu metallization based on 32nm and 22nm design rules are utilized for the experiments. All wafers are polished with an acidic aluminabased Cu slurry and a silica-based barrier slurry of basic chemistry. Various post-CMP clean chemicals are tested in the cleaning module. Among them, chemical A is acidic, while chemical B is basic. These chemicals are evaluated in various cleaning processes with different sequences as listed in Table I. BR1 and BR2 refer to the cleaning steps in roller brushes 1 and 2 with chemical spray. The “rinse” step is conducted in a tank filled with the clean chemical being constantly replenished in a pre-set bleed-and-feed cycle. During the rinse, the wafer is spinning at a constant rpm without mechanical contact with a brush or other component. Regardless of the cleaning processes, consumable lifetimes for pads, roller brushes, and slurry batches remain unchanged, i.e., P1, P2, and hybrid processes share the same consumable lifetime.
The CMP defects generated from the P1 and P2 cleaning processes are summarized in Fig. 3. With the acidic P1 process, PR/FM defects are low but HM and DE are high. As a consequence, extensive queue time control has to be enforced to reduce these corrosion-related defects, as reported in our previous study.2 On the other hand, the P2 process, with its basic chemical in both roller brushes, shows low HM and DE but high PR/FM defects. The result suggests that, compared with the acidic P1 process, the basic chemistry in the P2 process provides better passivation of the Cu surface to prevent the formation of corrosion defects. However, there is a certain deficiency in cleaning up the PR/FM and abrasive particles in the P2 process as shown in Fig. 4. Also observed is that in the P1 cleaning process, no circular ring defects are detected even after thousands of wafer passes. On the other hand, circular ring defects occur sporadically with the P2 cleaning process. The occurrence of circular ring defects, as depicted in Fig. 1, exhibits no dependence upon the lifetime (i.e., wafer passes) of the polish pad or roller brushes, nor does it correlate with the pot life of slurries or clean chemicals.
Table I: The cleaning processes and their process sequence evaluated in this study. Chemical A is acidic while Chemical B is basic.
Sequence: Process P1 P2 Hybrid
1 BR1 A B A
2 BR2 A B A
3 Rinse n/a n/a B
4 IPA drying Yes Yes Yes
B. Hybrid Clean Process To reduce all types of CMP-related defects, a new cleaning process is needed to remove all the PR/FM and abrasive particles while passivating the Cu long and sufficiently enough to prevent HM and DE defects until the cap layer is deposited. For that purpose, a hybrid clean process is developed whereby chemical A and chemical B are both included in the cleaning module with a specific process sequence. As listed in Table I, the hybrid process sprays the acidic chemical A in BR1 and BR2 and then applies the basic chemical B to rinse the wafer.
III. RESULTS A: All-acidic and All-basic Post-CMP Cleaning Processes Density of various CMP defects: P1 vs. P2 clean Defect density (arbitrary unit)
14 12 PR/FM
10
HM DE
8 6 4 2 96 wafers in 25 lots
113 wafers in 35 lots
0 P1
P2
Cleaning process
Fig. 3: CMP defects from two different post cleaning processes, P1 and P2 as described in Table I.
Fig. 5: Weighted defect density (WTDD) of major CMP-related defects between the P2 and Hybrid clean processes. The numbers on x- axis denote the sample size. (a) AL defects; (b) SH (silica abrasives), FM, and PR defects; (c) line-end hollow metal (LE), hollow metal (HM), and dendrites (DE); and (d) polish scratches (PS), handling scratches (HS), and light scratches (LS).
Fig. 4: Cu stacked wafer maps showing PR, FM and clusters of abrasive particles on Cu wafers cleaned with P2 process (left); and P1 process (right).
The cleaning performance of the hybrid clean process is compared with the basic P2 process and the results are depicted in Figs. 5(a), (b), (c), and (d). In this case, the data are
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collected over the course of ~ 3 months through multiple cycles of consumable changes (e.g., changes in pads, brushes, and slurry batches). Residual alumina abrasive particles (AL defects) from the Cu slurry are reduced by ~ 70% with the hybrid clean process as shown in Fig. 5(a). Similarly, as demonstrated in Fig. 5(b), silica abrasives (SH, from the barrier slurry), PR, and FM defects decrease remarkably with the hybrid clean process.
circular ring defects have been implementation of hybrid clean.
turned
off
by
the
CMP defects from the P2 and hybrid clean process are compared at multiple metal levels in Fig. 7. In this case, 300mm wafers based on 22nm design rule are processed with the two cleaning processes and their defect performance is monitored from metal 1 (M1) to metal 5 (M5). Over two months of data are collected and summarized in Fig. 7. The data clearly demonstrates that the newly developed hybrid clean process reduces CMP defects significantly and consistently.
Interestingly, as demonstrated in Fig. 5(c), corrosion-related defects such as HM, LE (line-end hollow metal) and DE are also reduced significantly by implementing the hybrid clean process. The deficiency of acidic chemical (i.e.., Chemical A) in preventing corrosion defects seems more than compensated by the final rinse step with basic Chemical B in the hybrid clean approach. It is possible that the presence of AL, SH, PR, and FM defects may have facilitated the corrosion process by introducing additional electrical charges that alter the local electrochemical potential across Cu wires. Therefore, with these AL, SH, PR, and FM defects removed from Cu surface by the acidic brush clean step, the final basic chemical rinse in the hybrid clean process can suppress the corrosive attack more effectively and reduce HM, LE, and DE defects. The basic chemical rinse step also cleans up some of the remaining AL, SH, PR, and FM defects before the wafer moves on to the drying step. Also observed is that with the hybrid clean process, scratch defects such as PS, HS, and LS are all reduced by large margin as shown in Fig. 5(d). In this case, the defect code HS includes handling scratches as well as circular ring defects. Since all the wafers share the same polish process, the difference in the scratch defect signal can only be the consequence of cleaning. The result suggests that a large portion of the scratches are generated not by the polish action on the platen, but by the spinning and scrubbing actions within the brush clean module. Once again, the removal of PR, FM, AL, and SH defects in the brush cleaners should be the key to the reduction of scratch defects as the hybrid clean process has demonstrated. The wafer maps in Fig. 6 clearly illustrate the differences in the effectiveness between P2 and the hybrid clean process.
Fig. 7: WTDD of CMP defects at multiple metal levels (from M1 to M5): P2 vs. Hybrid processes. Each bar represents the mean of two months of data.
Fig. 8: At-level short yield and next-level open yield with the P2 vs. hybrid clean processes.
Fig. 6: Stacked wafer maps showing PR, FM, abrasive particle, and PS defects on Cu wafers cleaned with P2 process (left) showing circular ring of scratches; and hybrid clean process (right) without circular ring.
When the hybrid clean process is implemented, no event of circular ring defects is reported, even after 6 months of process qualification and high-volume production. Essentially, the
Fig. 9: SEM review drive-back revealing a missing pattern (MP) defect at M2 caused by a prior-level (M1) PR defect underneath. In this case, the MP defect at M2 leads to loss in open yield.
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patterns on the wafer edge bevel area, where discontinuity of film deposition and litho patterns occur.
The remarkable CMP defect reduction achieved by the hybrid clean also translates to yield gain as shown in Fig. 8. Compared with the performance of P2 clean, both at-level short yield and next-level open yield are improved by implementing the hybrid clean process.
At the end of Cu polish, these residual AlOx abrasive particles are likely already trapped in the edge dummy patterns. The barrier polish may remove some but not all of them. The hybrid clean process does help reduce them by large margin, relative to the P2 process, yet there is still a high concentration remaining. Further reduction in these residual AlOx particles on the wafer edge may be achievable by implementing a bevel clean process with proper chemical such as the acidic Chemical A as described in Table I.
The improvement in at-level short yield should be the consequence of reduction in PR, FM, PS, AL, PS, and LS defects – all of them can cause short yield loss by introducing short current conducting path on the Cu surface. In addition, these surface defects become embedded particles when cap and ILD layers are deposited on top of them. They can create local topography variation, interfere with the photolithography process, and consume the depth of focus window, leading to patterning issues at the next level. One such example is illustrated in Fig. 9, where a PR defect at M1 results in a missing pattern defect and open yield loss at M2. Similarly, other surface defects such as FM, AL, SH can all induce MP defects and become open yield detractors at the next level. Therefore, by reducing PR/FM/AL/SH defects on the surface, hybrid clean process can actually improve open yield at the next level as shown in Fig. 7. This point will be elaborated on later. C. Surface Characterization Chemical and material characterization is conducted in order to understand the performance of the hybrid clean process. Total reflection x-ray fluorescence (TXRF) is employed to determine the existence of trace metals (e.g., Al) on wafers processed with different cleaning processes. Table II: TXRF Al metal density on blanket and patterned wafers processed with different cleaning processes. “Edge” refers to the ~ 5mm band on wafer edge while “center” represents the rest of wafer. Trace of Al by TXRF analysis Wafer and CMP cleaning process a. Blanket Cu, Hybrid
Al density (arb. unit) Center
Edge
0.36
0.25
0.4
0.26
c. Blanket Cu, ref (no CMP process)
0.38
0.33
d. 22nm patterned wafer, P2
1.23
100
e. 22nm patterned wafer, Hybrid
0.48
5.46
b. Blanket Cu, P2
Fig. 10: (a) (Top) Nano-sized particles embedded in the dummy pattern area located ~ 2mm away from wafer edge; (b) (bottom) EDAX analysis suggests these particles are AlOx abrasives from Cu slurry.
As shown in Table II, TXRF scans reveal no difference in the Al signal on blanket Cu wafers. Al density on wafers processed with the P2 or Hybrid clean is about the same as that from the reference wafer, i.e., as plated and annealed without a CMP process. However, on 22nm patterned wafers, significantly higher Al density is detected, especially within a band 5mm in from the wafer edge. In this case, the hybrid clean process reduces the Al signal by up to 20 times compared with the P2 process. SEM inspection and energy dispersive xray spectroscopy (EDX) analysis are performed to determine the nature of the high concentration of Al on the wafer edge. This analysis, shown in Fig. 10, reveals that the high Al signal is the result of AlOx abrasive particles embedded in the dummy
Fig. 11: XPS spectra on Cu surface treated with P2 and Hybrid clean process.
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X-ray photoelectron spectroscopy (XPS) spectra on wafers processed with P2 and hybrid clean processes are shown in Fig. 11. There is no distinguishable difference in the Cu surface state between the P2 and hybrid clean processes. Their spectra overlap, both showing distinctive Cu peaks at 932.5 eV (2p3/2) and 952.3 eV (2p1/2). The result suggests that, despite the usage of acidic P1 chemical in the roller brushes, the hybrid clean process passivates and protect the Cu surface at least equally well, compared with the basic P2 process. The result implies that the large amount of HM defect reduction realized by hybrid clean process can not be attributed to better passivation of Cu by the chemistry. Rather, it may have originated from the removal of surface defects (e.g., PR/FM/SH/AL) which could have expedited the formation of HM defects, as discussed before.
stability in the FOUP ambient conditions until the cap layer is deposited in order to prevent the formation of time-dependent Cu dendrite defects. Last but not the least, the clean chemical itself should be benign enough and not interact with residual chemicals from prior process steps (e.g., RIE, liner deposition, and plating…etc.) to create an aggravated environment for Cu corrosive and hollow metal formation. Previous work on post-Cu CMP cleaning process includes the optimization of contact kinetics during brush clean to enhance cleaning efficiency and to reduce brush scrubbing scratches.4,5 Regarding the clean chemical, the addition of inhibitors, surfactants, or chelating agents is common practice in the industry.6, 7 In the current study, we exploit the nature of chemicals to tackle the root cause of CMP defect generation without modifying the chemicals themselves. In general, metal oxides tend to dissolve in an acidic environment. Therefore, the use of acidic clean is a favorable choice for the purpose of PR/FM/AL/SH removal. In the case of hybrid clean process, the use of acidic chemical serves as the 1st stage of cleaning and the key to the removal of circular ring defects. The dissolution of CuOx and AlOx particulates with the acidic chemistry during brush clean eliminates the source that would otherwise generate the circular ring defects in the brush.8
IV. DISCUSSION A: Working Mechanisms of Hybrid Clean Process The newly-developed and production-proven hybrid clean process has demonstrated superior cleaning performance to conventional all-acidic or all-basic cleans with roller brushes. The new process combines the merits of both acidic and basic clean chemistries for the total reduction of most CMP-related defects including PR, FM, AL, SH, PS, and even HM. In addition, the hybrid clean process eliminates the occurrence of circular ring defects and exhibits no degradation in performance with consumable life times.
B: The Roles of Basic Chemical Rinse in the Hybrid Clean Process The basic chemical rinse in the hybrid process serves as the additional step to remove PR/FM/AL/SH even further. In this case, without the contact of roller brushes (hence no chance of generating circular ring defects), these surface defects are removed simply through undercut or lift-off mechanism in the repulsive zeta potential. As a matter of fact, in the basic chemical environment (pH > 10), the zeta-potential is negative on CuOx (i.e., Cu wafer surface) as well as on alumina and silica abrasives. Consequently, there is a built-in repulsive potential between Cu wafer surface and these residual abrasives in the basic chemical by nature, which can facilitate the removal of these PR/FM defects. The basic chemical rinse step needs to passivate the Cu surface to prevent the formation of HM and DE defects. Once again, basic clean chemical is a better choice for this task because Cu surface is passivated and protected in the high pH regime, according to Pourbaix diagram.
At the end of the barrier polish, a thin layer of oxide grows on Cu surface due to the usage of an oxidation agent, e.g., H2O2, in the slurry. In addition, the wafer surface is loaded with debris, abrasives, and polish residues; many are in form of oxides, such as AlOx, SiOx, TaOx, or SiCOx. These oxides are much harder than Cu and will scratch the Cu surface when the wafer is placed in close contact with the rotating roller brushes, leading to scratch defects and, worst of all, circular ring defects as shown in Fig. 1. Meanwhile, between the end of the barrier polish and the beginning of the IPA drying process, the Cu surface is exposed to the wet chemical environment for an extended time. As a consequence, there is high risk of corrosive attack and the formation of hollow metal defects if the post-clean chemical environment is too aggressive with respect to the Cu surface, or if it interacts with residual chemicals carried over from prior process (e.g., post-RIE etching or Cu plating).
It should be noted that the use of acidic and basic clean chemicals in sequence does raise the potential concerns of pH shock and the possible interactions between the two different chemicals to regenerate PR/FM defects. Intermediate steps such as a DI water rinse can be built into the process sequence to prevent such adverse effects. Therefore, the selection of basic chemical in the final step should take into consideration its compatibility with the acidic brush clean chemical.
Given this scenario, an ideal post Cu CMP cleaning process flow should begin with a clean chemical that creates a repulsive zeta-potential between the PR/FM defects and the surface of PVA brush to facilitate the removal of these defects as soon as possible before they can scratch the wafer to result in circular ring defects. A component that facilitates particle dissolution and/or light Cu etching should be included in the chemistry in order to enhance the cleaning efficiency. In addition, the clean chemical should protect the Cu surface by forming a thin yet coherent passivation layer to prevent or mitigate corrosive attack before the wafer is transferred to the IPA drying station. The passivation layer should also maintain its integrity and
C: Post-CMP Cleaning and Defect Characterization Also worth mentioning is that the vast reduction of CMPrelated defects not only improves the at-level short and open yield as demonstrated in the current study, but also helps
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reduce other defects not directly related to CMP at the next level.
TXRF scans reveal the concentration of residual AlOx abrasives on the wafer edge is greatly reduced by the hybrid clean process. The use of acidic clean chemical in the brushes to dissolve metal oxide is the key to the reduction of PR/FM/AL and the elimination of circular ring defects. The application of basic chemical rinse step provides further reduction in surface defects and passivation of Cu surface to prevent the formation of HM and DE defects. The new clean process helps improve short yield at current level and open yield at next metal level. It also reduces other defects such as missing patterns and non-visuals at next levels for better characterization and representation of other defects. ACKNOWLEDGEMENT This work was performed at the IBM Microelectronics Div. Semiconductor Research & Development Center, Hopewell Junction, NY 12533. One of us (W.-T. Tseng) would like to express gratitude to Mr. Ricky Hull and Mr. Aquileo VasquezHerrera, both of IBM, for their technical assistance throughout the course of the work. REFERENCES
Fig. 11: SEM drive-back shows that the existence of a PR defect at M1 level leads to a non-visual (NV) defect at the next level (top); similarly, a PS defect at M1 “shows up” as a NV defect at M2 (bottom).
[1]
Furthermore, surface defects such as PR and PS can still be detected and they will “appear” as non-visual (NV) defects at the next level as illustrated in Fig. 11. At the first look, the appearance of these NV defects would seem nuisance in nature. Nevertheless, in fact, their existence can be the indicative of surface defects underneath (i.e., at prior level) and therefore potential sites for circuit failure. From characterization and defect inspection point of view, the reduction in NV defects also translates to better detection and representation of other real defects. Therefore, the implementation of hybrid clean or any other CMP defect reduction measures to remove surface defects has the additional advantages of reducing other defects indirectly related to CMP, e.g., MP and NV, and assisting defect inspection and classification for characterization purpose.
[2]
[3]
[4]
[5]
[6]
V. CONCLUSION [7]
A post-Cu CMP hybrid clean process is developed and implemented for advanced BEOL technology. The process combines acidic and basic cleans in sequence and achieves a more than 60% reduction in all CMP related defects, including polish residues, foreign materials, slurry abrasives, scratches, and hollow metal, relative to an all-basic clean process. It also eliminates the occurrence of circular ring defects that are generated during roller brush cleans.
[8]
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